Are Short Cranks Faster?

According to some of the world’s best triathletes, shorter cranks can help you achieve lower drag and faster run splits.

It may surprise you that Craig Alexander, at 5’11” (relatively tall for a professional triathlete) used short 167.5mm cranks on his bike when he won the 2011 Ironman World Championships in Hawaii.

Traditionally, bikes are sold with cranks that are relative in length to the size of the frame. So a small frame will probably have 170mm cranks, a medium will have 172.5mm cranks and a large will most likely be fitted with 175mm cranks. This made sense once and has never really been questioned until recent years and the explosion in popularity of triathlon and the increase in the number of time trial bikes on the road. Recent studies have now shown that the power loss from shortening your cranks is minimal but that there could be tangible aerodynamic benefits to your bike position and your ability to run freely off the bike. This thinking has quietly found its way into the set-ups of several top pro triathletes including Craig Alexander and Julie Dibens – who have experimented with shorter crank lengths in the last two years and noticed improved performances.

Here to help you decide if you would benefit from shorter cranks is the owner of freespeed.co.uk and a Retul certified bike-fitting studio in London, Richard Melik.

Short Cranks

How to Ride Faster

Let us assume that you already have an excellent position on your bike and are currently using 175mm long cranks. If you decided to swap your 175mm cranks for shorter 165mm cranks, the following would happen: your pedal would end up being 10mm closer to your saddle when it’s at the bottom of the pedal stroke, compared to before. To adjust for this you would then need to raise your saddle by 10mm to give yourself the same leg extension through the bottom of the pedal stroke as before. The key benefit is that now when your pedal is at the top of the pedal stroke it is 10mm further away than before, plus your saddle is already 10mm higher. This gives you 20mm more space between your knees and your upper-body at the top of the pedal stroke. In other words, you can now ride with a more open hip angle, without your knees coming so close to your chest. From here you have two choices. You can either enjoy the delights of riding with a more open hip angle or you can lower the front end of the bike by up to 20mm and effectively return to your previous hip angle, but with a flatter back and potentially improved aerodynamics.

How to Run Faster

Your hip angle doesn’t just impact your comfort and aerodynamics either – it can impact your running too. This is because six or seven hours in the saddle with a cramped hip angle caused by an aggressive back angle will negatively impact your running ability. Put another way, tight hip flexors and a stiff back will not allow you to run quickly and fluidly. Keeping your hip flexors relaxed and fresh for when you exit T2 will pay dividends in a better run split and faster overall time.

Are They For You?

Short cranks are not for everyone. If your bike position is relatively conservative with
a high back angle then the benefits from shorter cranks will be negligible. In fact, you may feel worse as more force is required to pedal the same gear using shorter cranks. However, if you are racing at the sharp end and are keen to maximise your aero advantages then shorter cranks will allow you to go lower and be more aerodynamic at the front of the bike without compromising your hip angle.

Before you rush to your local bike shop, there are certain factors to consider before you make such a significant change. Several variables such as age, aspirations, flexibility and injury history should be taken into account. Just because pro triathletes like Chris Lieto are comfortable with a hip angle of, say, 45 degrees, doesn’t mean it’s right for someone who doesn’t have the same conditioning. Think carefully about your personal limiters – for an Ironman-distance event, increased comfort and an open hip angle might mean you race faster, whereas if you’re attempting to set an Olympic-distance PB, a flatter back and improved aerodynamics may be the magic formula.

There are very few downsides to shorter cranks other than they can feel a little weird if you have cycled thousands of miles on your regular length cranks. You may also notice that your gearing feels a little harder than normal, this is due to the decreased leverage that comes from a shorter crank arm. So should you make the jump? Different cranks are a significant investment, as you will need to fit them to all of your bikes. If you are currently comfortable on your bike and performing well then it may not be the time to experiment. If, however, you are struggling with feeling cramped at the top of the pedal stroke or if you feel your position is not as aerodynamic as you would like it to be then a move to shorter cranks may unlock faster bike and run times.Aerodynamics- Is lower always better

Drag is caused by a combination of factors that determine the airflow around your body and bike. Lowering the front end of your bike (to achieve a flatter back) helps to shrink your frontal profile and reduce your drag. However, forcing the body too low often makes the rider put their head up and so increases the size of their silhouette